1. Field of the Invention
The present invention relates to a code synchronous timing determining method upon information reception in a CDMA (code division multiple access) system, a determining apparatus thereof, and a CDMA information signal receiving apparatus.
2. Description of the Related Art
In the CDMA system, a transmission information signal of a specific channel can be received at a receiving station by de-spreading the transmitted spectrum spread signal using the same code data as code data used at a transmitting station in which the transmission information signal is spectrum spread by the code data. At the time of reception, the code synchronization must be established between the code data included in the transmission information signal and the code data prepared in the receiving station.
When an information signal is transmitted via the CDMA system, the length of a CDMA wave transmission path from the transmitting station to the receiving station is varied due to the change of the wave propagation direction by tall buildings and the like. The propagation direction of the CDMA wave is changed by a plurality of buildings many times so that the CDMA wave is propagated through a plurality of different propagation paths or multiple paths.
As a result, the same CDMA information signal is received at the receiving station at plural different timings depending on the difference in the path length. Also depending on the difference in wave propagation speed due to difference in temperature or density of air which is a propagation medium of the CDMA wave, plural CDMA waves of the same content are received at the receiving station at plural slightly different timings.
At the CDMA wave receiving station, as mentioned above, the correlation of the both code data is calculated by using the same code data as the code data used in spectrum spreading at the transmitting station to detect the timing at which a high correlation is detected, whereby the code synchronization is established and the CDMA reception signal in the assigned channel is de-spread and demodulated to regenerate an information signal transmitted from the transmitting station.
When the CDMA wave transmitted from the transmitting station is propagated through the multiple paths, plural code synchronization timings each having a high code correlation to a transmitted information signal in the assigned channel should be established.
In the CDMA wave receiving station, a combining device for accumulating a plurality of information signals propagated through multiple paths is used. In the combining device, the information signals incoming from other paths than the shortest path giving the largest correlation are combined in addition to the information signal obtained from the shortest path to enhance the reception precision. Such reception method is known as RAKE reception method in the CDMA communication field. In this RAKE reception method, the information signals propagated through multiple paths are utilized, and in order to enhance the information processing efficiency, the number of path information signals to be selected must be limited to a specified number including that giving the largest correlation. That is, the paths to be used should be limited to a finite number.
When receiving CDMA information signal, a code correlation value is calculated at plural sampling timings in a period corresponding to one itinerary time of code series used in spectrum spreading, and these code correlation values are stored, for example, in a RAM by using the address corresponding to each sampling timing. To utilize the received information signals incoming from a specified number of multiple paths in the RAKE combining device, it is required to obtain data of high correlation value timings of the number corresponding to the specified number of multiple paths.
In order to obtain a high correlation value timing, all sample timings in one itinerary time of code series are scanned, that is, from the beginning address to the final address in the RAM, and the largest correlation value of the read correlation values is determined. By extracting the timing corresponding to the address of the RAM conforming to this largest correlation value, it is determined as first candidate timing data for code synchronism of the received wave signal propagated through one of the multiple paths.
In the conventional timing determining method, when obtaining second candidate timing data for a path other than the one path giving the first candidate timing data, too, while masking data corresponding to the first candidate timing data determined in the first place, all sampled correlation values in one code series time are scanned in the RAM, that is, from the beginning address to the final address in the RAM, and the timing of the address corresponding to the next largest correlation value of the already determined the most largest correlation value is extracted, and obtained as the second timing data.
Similarly, thereafter, in all timing data for the specified number of multiple paths, all sampled correlation values in one code series time are scanned, that is, from the beginning address to the final address in the RAM, and the timing corresponding to the subsequent number of large correlation values of the remaining correlation values stored in the RAM is extracted sequentially, and obtained as the respective timing data. It hence takes a long time to establish the code synchronism of reception signals in each finger circuit of the RAKE combination device.
Incidentally, one path giving the timing to show a largest correlation value may have a plurality of timings giving large correlation values at close timings slightly deviated from this given timing. In such a case, even if one or more large correlation values are obtained, such vicinal timings should be excluded as candidates for specified timing data.
At the time of such data exclusion, however, a true candidate which should not be excluded for specified timing data may be excluded by mistake. Thus, to avoid such incident, a redundant storage device for temporarily storing all the corresponding data is needed.
Moreover, when deleting undesired timing information for exclusion, it is required to monitor the change state of the stored correlation value by time, and much complicated deletion control is necessary.
Recently, to enhance the reception performance of CDMA signal, there is a tendency of narrowing the sampling interval when extracting code correlation values. Therefore, when the spectrum spread rate of transmitted information signal is large, there are too many candidate timings to be scanned. In such a case, the number of timings of large correlation values to be detected is plenty, and it takes a longer time until the code synchronism is established for each of the multiple paths.